This invention relates generally to composite cylinders used in injection molding or extrusion operations, and more particularly to the composition of the inner lining of such composite cylinders.
Injection molding is a process for forming articles from plastics, wherein a heated, fused plastic is forced under pressure into a die cavity to solidify in the shape and size of the cavity. The charge of plastic material to be injected into the die cavity must be heated and pressurized prior to injection, and in one approach the plastic starting material is fed into a hollow cylinder having a screw therethrough. As the screw turns, the plastic is forced into a heated zone of the cylinder ahead of a check ring on the head of the screw, so that a predetermined volume of heated, pressurized plastic is prepared for subsequent injection into the die cavity by a forward movement of the screw and check ring within the cylinder.
Because the economics of injection molding depend upon attainment of long operating lives for the machinery, it is important that the inside of the cylinder have high resistance to wear and corrosion by the heated plastic material. Should the inside of the cylinder wear away so that the inner diameter of the cylinder is enlarged, a clearance develops between the check ring and screw, and the inner wall of the cylinder so that the plastic material leaks back from the pressurized zone, with the result that the necessary pressure for injection molding will not be developed. The cylinder must then be refurbished or replaced, or a larger diameter check ring must be utilized, and in any event the economic production process is interrupted.
Two principal approaches have been taken to provide cylinders having highly wear-resistant and corrosion-resistant inner linings, while at the same time having an outer housing portion with high strength and toughness. In one, the inside of a steel cylinder is nitrided, as by exposing the inner wall of the cylinder to a nitrogen-containing gas such as ammonia, at elevated temperature. This nitriding process results in a relatively thin layer of hardened steel on the inside of the cylinder, having improved wear-resistant properties.
An improved inner layer is achieved by making a composite cylinder using a centrifugal casting process, wherein ingredients suitable for forming an inner layer within the cylinder are loaded into a premachined cylindrical outer steel housing, the ends are sealed, the housing is placed in a furnace at a temperature sufficiently high to melt at least a portion of the ingredients but not the housing, and the cylindrical housing is then rotated rapidly about its cylindrical axis to distribute the molten ingredients in a continuous layer about the inside of the housing. Upon cooling, the inner layer is metallurgically bonded to the cylindrical outer housing, and the inner layer may then be machined or honed to form a smooth bore of constant diameter to receive the screw and check ring.
The centrifugal casting process has proved successful for the manufacture of composite cylinders for injection molding, and a number of alloys have been developed for use in forming the inner lining of the cylinder. Among the materials developed is a heavy-metal alloy of tungsten-carbide particles in a nickel-chromium-cobalt based matrix, wherein the tungsten carbide imparts hardness to the inner layer, and the matrix material imparts toughness and allows the fabrication by the centrifugal casting process. In such alloys developed to date, the amount of tungsten carbide has been limited to a maximum of about 45 weight percent of the total alloy, even though higher tungsten-carbide contents should improve the wear resistance. With such alloys, if the percentage of tungsten carbide is above about 45 weight percent, the matrix material cannot melt and flow properly and an irregular, lumpy, porous inner lining is formed. Accordingly, there has been a need for an improved alloy utilizing higher volume fractions of tungsten carbide to give improved wear resistance to the lining of cylinders for injection molding or extrusion machines. The present invention fulfills this need, and further provides related advantages.